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6.2 Raghunath Subedi – University of Copenhagen

ReUseWaste > Thesis abstracts > 6.2 Raghunath Subedi

WP 6.2: Land utilization, crop nutrient value and green house gas emissions from digestates and compost based bio-fertilizers, Raghunath Subedi, UTO

Climate change represents the global threat to the human and animal life and the issues are becoming catastrophic, seeking additional mitigation practices in order to off-set the risk of irreversible effects. Both animal manures and crop residues are bulky materials containing essential elements, while their mismanagement would result not only in loss of nutrients but also significant negative impacts to the environment. Pyrolysis of these waste materials can offer a potential option in the manure processing chain through biochar and bioenergy production, nutrients recovery, and reduction in manure volume and cost for transportation.

Biochar obtained from the pyrolysis of crop residue and manure is a potential solution to reduce greenhouse gases (GHGs) emissions, increase carbon (C) sequestration, and as a beneficial soil amendment. This study aims to evaluate and understand the soil application effects of crop waste- and manure-derived biochars on ammonia (NH3) and GHGs emissions, nutrient availability, crop growth and soil quality. In order to achieve the objective, three separate experiments with different periods were carried out in laboratory conditions.

The first experimental study measured potential emissions of NH3 following application of pig slurry to the surface of silt-loam and loam soils amended with miscanthus-derived biochar and hydrochar at a rate of 3% soil dry weight and 60% water-filled pore space (WFPS) over 48 hours period. Results showed that hydrochar amendments significantly increased (p<0.05) emissions of NH3 compared with the biochar and Control treatments for both soils; cumulative NH3 emissions averaged 38.7% and 23.5% of applied total ammonium nitrogen (TAN) for hydrochar and biochar, respectively, whereas it was 18.2 % for the control. The greater emissions in hydrochar-amended soil were linked to the reduced ability to adsorb ammonium (NH4+) associated with greater hydrophobicity and strong pH buffering of the slurry. The lower soil NH4+-N concentrations with biochar amendment compared to the Control suggested that it became immobilized by adsorption onto biochar surfaces.

The second experiment quantified the emissions of CO2 and N2O and assessed soil chemical properties following amendment with four different manure-derived biochars produced from different feedstocks (poultry litter (PL) and swine manure (SM)) at different temperatures (400 or 600 °C) and a commonly available standard wood chip (WC) biochar, produced at a high temperature (1000 °C) in silt-loam and sandy soils during 90 days incubation. The cumulative C mineralization and the emissions of N2O with the PL400 and SM400 treatments were significantly higher (p<0.05) compared with the Control and the WC biochar treatments in both soils, and were positively correlated with the volatile matter (VM) and nitrogen (N) contents of the biochar. Soil nutrient availability was greater in both soils in which the manure-derived biochar was used compared to the WC biochar, but were less effective in C sequestration compared to the standard WC biochar.

The third experiment assessed potential of manure-derived biochars in promoting plant growth and enhancing soil chemical and biological properties in silt-loam and sandy soils during a 150 days pot experiment. The PL400 and SM400 biochars significantly increased (p<0.05) shoot dry matter (DM) yields (silt-loam soil) and enhanced N, phosphorus (P) and potassium (K) uptake in both soils, compared to the Control. All biochars significantly contributed to increasing the soil organic carbon (SOC) contents compared to the Control. The intracellular enzyme- dehydrogenase activity (DA) was significantly enhanced for PL400 and SM400 treatments and was positively correlated with the volatile matter VM contents of the biochars, while the opposite case was true for the extracellular enzyme: β-glucosidase activity (GA). The 16S rRNA gene based bacterial community structure was significantly affected (p≤0.05) due to all biochars treatments compared to the Control in both soils.

This PhD research study demonstrated that biochar properties varies greatly based on feedstock type and pyrolysis conditions, and therefore their resulting effects on gaseous emissions, soil properties and crop response also varies with individual soil-biochar combinations. The negative effect of miscanthus bio- and hydro-chars amendments on NH3 emissions following application of pig slurry seeks further investigation with advanced methodological approach. Manure biochars showed a potential to be utilized as both soil conditioner and organic fertilizer, while simultaneously sequestering C in soils. On the other hand, the WC biochar, besides its high C sequestration potential, can show positive effects on soil and crop only if applied in combination with NPK fertilizers.

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